Carbon dioxide (CO2) geological storage is one of the approaches considered for stabilizing atmospheric CO2 concentrations. Captured CO2 from a source such as effluent from a coal-fired power plant is injected through a well into the subsurface, e.g. saline aquifers. Once injected, CO2 is expected to be confined by overlying and underlying impermeable layers in the reservoir, enabling long-term (thousands of years) storage whether the stored form is as a separate CO2-rich phase or in the form of carbonate minerals or as dissolved solute.
In order to sequester carbon dioxide exiting a power plant into underground reservoirs, it is necessary to separate the carbon dioxide from the flue gas, and compress it before pumping it underground into the reservoir. The separation and compression steps typically entail a 15-30% penalty in the net power delivered by the power plant. In addition, the purity levels for the sequestered CO2 may be required by legislation to be 99+% which would entail prohibitive power penalties. Conversely, in order to improve power plant efficiencies, it may be necessary to accept contaminant components in excess of 1% (by mole) in the CO2 flow stream.
It has been shown that changes in fluid composition may affect the design and operation of surface facilities and pipeline networks to deliver the gas at certain fixed wellhead conditions for injection. U.S. Pat. Nos. 4,835,687 and 6,701,223 and patent publication US2007/0260333 relate to the monitoring and computational control of pipeline parameters for power optimization in natural gas streams. U.S. Pat. No. 6,201,163 and patent publication US2001/0007915 relate to the addition of less compressible but miscible hydrocarbons to natural gas streams in order to lower pipeline compression power consumption. Patent publication US2006/0254287 relates to optimization within methane-rich natural gas systems.
Standard equations for calculation of pressure drop in pipelines for single-phase flow are given below. For compressor calculations, the work done for adiabatic (or isoentropic) compression is divided by an efficiency factor. The power required to compress gases is given by the following formula:
                              W          ad                =                              2.78            ·                          10                              -                4                                              ⁢                      k                          k              -              1                                ⁢                                    WRT              1                        9806                    ⁢                      ⌊                                                            (                                                            P                      2                                                              P                      1                                                        )                                                                      k                    -                    1                                    k                                            -              1                        ⌋                                              (        1        )            where Wad is power in kW, W is mass flow rate in kg/s, R is the universal gas constant (J/kg/K), T1 is the upstream temperature (K), P1 is the absolute upstream pressure (kPa), P2 is the absolute downstream pressure, and k is the ratio of specific heats for the gas (Cp/Cv). To calculate the actual power used in compression, Wad is divided by an efficiency factor (˜0.7). The downstream temperature is calculated by the following relationship for ideal gases:
                                          T            2                                T            1                          =                              (                                          P                2                                            P                1                                      )                                k                          k              -              1                                                          (        2        )            
With respect to heat exchanger calculations, the aftercooler, intercooler and the heater/cooler in a surface facility may either be air cooled, water cooled or refrigerant cooled. If the gas stream needs to be heated, the heat may be obtained from steam, electric power, combustion of a suitable fuel, or waste heat from a process. The energy exchanged may be calculated from the following equation:Q=W(h2−h1)≈WCp(T2−T1)  (3)where Q is the heat exchanged, hi is the enthalpy of the gaseous stream and Cp is the specific heat (at constant pressure) of the gas.
With respect to pipe flow calculations, there are three equations that need to be solved simultaneously. The first is the mass conservation or the continuity equation, the second is the momentum balance and the third is the energy balance:
                                              ⁢                                                            ∂                                  (                                      A                    ⁢                                                                                  ⁢                    ρ                                    )                                                            ∂                t                                      +                                          ∂                                  (                                      ρ                    ⁢                                                                                  ⁢                    u                                    )                                                            ∂                x                                              =          0                                    (        4        )                                                          ⁢                                                            ∂                                  (                                      ρ                    ⁢                                                                                  ⁢                    u                                    )                                                            ∂                t                                      +                                          ∂                                  (                                                            ρ                      ⁢                                                                                          ⁢                                              u                        2                                                              +                    p                                    )                                                            ∂                x                                              =                                    ρ              ⁢                                                          ⁢              g              ⁢                                                          ⁢              sin              ⁢                                                          ⁢              θ                        -                                          1                2                            ⁢              f              ⁢                                                          ⁢              ρ              ⁢                                                          ⁢              u              ⁢                                              u                                            ⁢                              S                A                                      -                          ρ              ⁢                                                          ⁢                              u                2                            ⁢                              1                A                            ⁢                                                ⅆ                  A                                                  ⅆ                  x                                                                                        (        5        )                                                                    ∂                              (                                  ρ                  ⁢                                                                          ⁢                  E                                )                                                    ∂              t                                +                                    ∂                              (                                  ρ                  ⁢                                                                          ⁢                  Hu                                )                                                    ∂              x                                      =                                                            k                T                            ⁡                              (                                                      T                    wall                                    -                  T                                )                                      ⁢                          S              A                                -                      ρ            ⁢                                                  ⁢            g            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            θ                    -                                    1              2                        ⁢            f            ⁢                                                  ⁢            ρ            ⁢                                                  ⁢            u            ⁢                                        u                                      ⁢                          S              A                                -                      ρ            ⁢                                                  ⁢            Hu            ⁢                          1              A                        ⁢                                          ⅆ                A                                            ⅆ                x                                                                        (        6        )            where A is the pipe cross sectional area given by A=0.25πD2, E is the internal and kinetic energy given by E=e+0.5u2, H is the enthalpy and kinetic energy given by H=h+0.5u2, S is the pipe perimeter given by S=πD, D is the pipe diameter, e is the internal energy, f is the Fanning friction factor (see Page 5-24, Chemical Engineers Handbook, 6th edition), g is the acceleration due to gravity, h is the enthalpy, kT is the heat transfer coefficient, p is the pressure, t is time, T is the temperature, Twall is i the wall temperature, u is the average fluid velocity, x is the distance along the pipe, θ is the pipe inclination with respect to the horizontal, and ρ is the density.